There have been proposed a variety of processes for producing reduced iron or an iron alloy. Of these, there is one known as a process of high productivity that includes the use of a rotary hearth reducing furnace, with which the reduction of a metal is effected. The essential part of this process is the use of a rotary hearth reducing furnace for reducing a metal oxide, the furnace being a firing furnace of the type in which, under a fixed fire resistant ceiling and side walls, a center-lacking-disk-shaped fire resistant hearth rotates along a rail at a constant speed (hereafter, referred to as a rotary furnace). Generally, the diameter of the disk-shaped hearth is from 10 meters to 50 meters, and its width is 2 meters to 6 meters.
After having been mixed with a carbon-based reducing agent, a metal oxide-containing powder as a raw material is pelletized to make raw material pellets and fed to the rotary hearth. The raw material pellets are spread all over the hearth. Since the raw material pellets are held stationary in relation to the hearth, this process has the advantage that the raw material pellets are hardly broken up in the furnace and therefore it is free from the problem of the raw material breaking up to form a powder which sticks to the fire resistant hearth. Moreover, this process has the advantage of high agglomerate production yield. Furthermore, since its productivity is high and a less expensive coal-based reducing agent and powdered raw materials can be employed, in recent years, the number of cases employing this method are increasing.
The rotary hearth method is also effective in reducing dusts generated through the process of steel manufacture in blast furnaces, basic oxygen furnaces, and electric arc furnaces, and in reducing thickener sludge generated in a rolling step, and in removing impurities therefrom. This method has been adopted for treating dusts and is effective in recycling materials.
How the rotary hearth method is carried out is described below in outline. Firstly, after metal oxides such as ores, dusts, or sludge as a raw material are mixed with a certain amount of a carbon-based reducing agent required for the reduction of the oxides, the resulting mixture is granulated into pellets of from several millimeters to several tens of millimeters by a granulator such as a pan pelletizer while supplied with water so as to have an average moisture content of approximately 10%. In the case where the particle size of an ore as the raw material or that of the reducing agent is large, after having been crushed by a pulverizer such as a ball mill, the materials are kneaded and granulated.
The pellets thus obtained are fed onto the hearth of a rotary furnace in layers. The pellets spread all over the hearth are quickly heated and calcined at temperatures as high as 1100 to 1300° C. for 5 to 20 minutes, whereby the metal oxide is reduced by the reducing agent mixed in with the pellets to provide a metal. Depending on what kind of metal is to be reduced, the degree of metallization varies. In the case of iron, nickel, or manganese, the degree of metallization reaches 95% or higher. Even if the metal to be reduced is chromium, the reduction of which is difficult, the degree of metallization reaches or exceeds 50%. Moreover, in the case of treating dusts generated by the steel manufacturing industry, their recirculation back to a blast or electric arc furnace is easy, because such impurities as zinc, lead, alkaline metals, and chlorine are volatilized as the reduction reaction proceeds.
As described above, in the method for reducing metal or dusts generated in the process of steel manufacture through the use of a rotary hearth, it is necessary to pelletize the raw material and the reducing agent. It is also essential that, as a pretreatment of the raw materials, the mixture of a powdered metal oxide and a reducing agent as the raw material be brought into a condition suitable for granulation. As the pretreatment, milling of the raw materials, kneading with a ball mill, or any of a variety of techniques are carried out.